What started World War I?

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The trigger for World War I was the June 28, 1914 assassination of Archduke Franz Ferdinand of Hungary by the anarchist Serbian Gavrilo Princip in Austria. However, the true cause of the war was a combination of romantic nationalism and a network of alliances that, while designed to balance European power, made war both inevitable and devastating in size.

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The complex network of alliances binding these nations together included the Treaty of London, the Three Emperors League, the Dual Alliance, the Triple Alliance, the Entente Cordiale, the Anglo-Russian Entente and the Triple Entente. Most of these alliances were not designed for military intervention, but rather to counter the growing power of one or another European nation, particularly Germany. Germany, only a century earlier a patchwork of warring principalities, had condensed into a powerful, militaristic country under Otto von Bismark during the 1870s, quickly rising to challenge the other European powers.

Princip's actions led to a cascade of countries calling in allies based on the alliance networks. Austria-Hungary had been looking for a reason to attack Serbia, but before declaring war, it ensured Germany would stand by its earlier treaty of mutual defense. Serbia, meanwhile, ensured Russia would support it. Russia ensured it had the backing of France and Britain. When war was finally declared a month later, most of Europe and, by extension, the worldwide imperial holdings of these countries, were already committed to the fight.